Electronically commutated motors, so-called EC motors, are frequently used as fan drives. These drive units generally consist of a permanent magnet excited synchronous motor (PMSM) with integrated power and signal electronics which frequently are external rotor motors.
Electric motors of this kind can be operated from a single-phase or three-phase AC voltage by first rectifying the AC voltage to an intermediate circuit voltage, which is then converted via a controlled inverter into a motor operating voltage to power and commutate the motor. As a rule, the inverter is controlled by a field-oriented current-space vector regulator, wherein a q-current as the torque-forming component of the current-space vector is regulated perpendicularly to the permanent magnet field in order to achieve a maximum torque. A d-current is controllable in the direction of the permanent magnet field and forms a field-influencing, i.e. field-forming or field-attenuating depending on the direction of the current, component of the current-space vector. In synchronous motors, the d-current is usually regulated to zero in order to reach optimum efficiency.
In order to achieve a motor torque with minimum ripple (ripple of the air gap torque, i.e. of the internal motor moment) as most uniform and constant as possible and thereby a favorable noise behavior (especially in fan drives), the intermediate circuit voltage should preferably be a constant DC voltage. In order to achieve this, the DC voltage rectified via a main rectifier and highly pulsating, has so far been smoothed via at least one smoothing capacitor and if necessary, via an additional filter choke. For this purpose, the smoothing capacitor actually has to have a relatively large capacitance (e.g. several hundred μF) so that standard electrolyte capacitors (Elkos) can be used. But in practical usage, the latter have several disadvantages, and specifically in particular a large installation volume and a short service life.
There is today therefore, an increasing tendency to either avoid the smoothing capacitors entirely, or at least to avoid the electrolyte capacitors, and, in the latter case, to use longer lasting foil capacitors with reduced capacitance (only up to several μF) are used. Owing to the absent, or only minimum intermediate circuit reactances, the term “slender intermediate circuit” has been used, wherein decoupling of the main and motor side by means of storage components, such as capacitors and chokes (reactances), is wholly or at least largely omitted. This means that a slender intermediate circuit contains no, or only minimum, intermediate circuit reactance.
With this technology of the “slender intermediate circuit” particular problems chiefly occur in the main single-phase power supply from the single-phase mains (standard main AC supply frequency e.g. 50 or 60 Hz), because the rectified DC voltage pulsates very strongly at twice the main frequency (e.g. 100 Hz or 120 Hz) between zero and a peak value, wherein the voltage profile corresponds to the value of the sinusoidal mains AC voltage. If an EC motor (PMSM) were now to be powered directly from such a highly pulsating DC voltage, only an insufficient motor current could be applied below a certain threshold voltage to the motor windings which would no longer be able to maintain the required torque constant.
It is the object of the present invention to optimize the operation of an electronically commutated electric motor (EC motor) with “slender intermediate circuit” in a technically favorable manner and with simple and cost-effective means.
According to the present invention, this object is attained by a method according to this invention. A suitable control system for applying the method is also described. Advantageous embodiments of the invention are presented in the following description.
According to the present invention, a dynamic field attenuation thus takes place by defining the d-current in the negative range with a sinusoidal profile and double main frequency, and without measuring the main and/or intermediate circuit voltage, and wherein the d-current is regulated according to its phase position (relative to the mains frequency) and its amplitude depending on the q-current such that a ripple of the q-current is minimized. Since the q-current as the torque-forming component is proportional to the torque, the torque ripple (ripple of the air gap torque=internal motor torque) is thus also minimized, and specifically in spite of the intermediate circuit voltage pulsating very strongly between zero and a peak value.
The invention is based on the fact that the d-current should ideally be dynamically regulated in such a manner that the current-space vector in the fixed-rotor coordinate system is always tracked so that the length of the voltage-space vector (corresponding to the amplitudes of the phase voltages) is always as long in the fixed-rotor coordinate system as is currently possible from the pulsating intermediate circuit voltage (tracking of the phase voltage). For this purpose, however, an exact solution of a differential equation and a corresponding technical implementation would be required. The present invention, however, attains that due to the d-current which is variable in time, the phase voltage is tracked on the basis of the time course of the intermediate circuit voltage, so that the resulting motor phase voltages can still be generated at sufficiently good approximation from the strongly pulsating intermediate circuit voltage. A negative d-current causes a field attenuation, which results in that the motor can be operated at a lower terminal voltage and thus a lower intermediate circuit voltage and still, at a then higher power consumption, produce its rated power (rated torque, rated speed). Therefore, despite the pulsating intermediate circuit voltage and even at small voltage values below the defined limiting voltage, the motor can be kept running at nearly constant torque due to the field attenuation. In addition, due to the dynamic field attenuation, a dynamic power storage occurs in the existing motor inductances and a resulting energy feedback into the intermediate circuit (namely advantageously without operating the motor by means of the generator which changes the torque) as well as a phase power smoothing. It virtually is a “boost converter effect,” as a result of which the intermediate circuit voltage is additionally increased and the torque ripple further reduced.
It should additionally be mentioned that a voltage-space vector with a maximum of half the length of the intermediate circuit voltage can always be generated from the instantaneous intermediate circuit voltage. The amplitudes of the three phase voltages in the fixed-stator coordinate system thus correspond to the length of the voltage-space vector in the fixed-rotor coordinate system. The phase voltages are formed from the intermediate circuit voltage on the basis of the voltage-space vector via transformation by means of pulse width modulation (PWM). Thus the maximum possible amplitude of the phase voltages corresponds to half the intermediate circuit voltage.
A large motor inductance promotes the voltage feedback via the d-current modulation. Likewise, it contributes to smoothing the motor currents and thus to keeping the torque constant. The reason for this is that the dynamic field attenuation allows operating the motor up to a minimum phase voltage and thus intermediate circuit voltage without collapse of the torque. The feedback of energy via the d-current modulation indeed increases the intermediate circuit voltage (boost converter effect), but not always up to the minimum voltage that can be achieved by field attenuation. The remaining voltage difference thus results in that the phase current briefly collapses (pulses) causing torque fluctuations. A large motor inductance, which has to be present anyway due to the large voltage drop caused by dynamic field attenuation, smoothes the pulsating current profile, just as any inductance smoothes out a current profile (energy storage), and ensures that the power flux can be kept nearly constant. From the approach of the differential equation, which leads to an exact solution of the d-current profile as a function of the intermediate circuit voltage, it follows that due to the dynamic field attenuation, the phase voltage and thus, as described, the intermediate circuit voltage can be reduced more than would be possible (at the same id-current peak value) by constant field attenuation, since the differential voltage drops are included in the phase voltage due to the rate of change of the d-current.
The torque-forming q-current is held constant independently of the d-current or defined by a speed controller when the inverter is actuated. This means that the dynamic field attenuation changing the d-current has no effect on the q-current, except for the ripple reduction, or on the respectively adjusted motor speed. Due to variation of the d-current, only the length and angle of the current-space vector resulting from the mutually perpendicular components changes.
A control system according to the present invention in the first instance consists of the standard components for EC control, namely a main rectifier and a downstream inverter connected via an intermediate circuit which is actuated to generate quasi-sinusoidal motor currents for corresponding voltage clocking (modulation) of a PWM control. For this purpose, means are provided for field-oriented current-space vector regulation with a speed controller to specify a q-current as the torque-forming component of the current-space vector. According to the present invention, the control system has a function generator to specify a dynamically changing d-current with a sinusoidal profile and a double the main frequency as current component for dynamic field attenuation and in addition, a two-dimensional extreme-value regulator which regulates the sinusoidal d-current according to its phase position and amplitude as a function of the q-current in such a manner that ripple of the q-current is minimized. The additional components of the control system, and specifically the function generator and the extreme value regulator, can be implemented in a relatively simple technical and cost-effective manner.